FR: Read /etc/docker/config.json in addition to ~/.docker/config.json

[ChristianTackeGSI]

As a sysadmin, I would like to set some options globally for all users.

So it would be good, if the logic reading ~/.docker/config.json would also read /etc/docker/config.json and merge the results.

When it comes to precedences, I think, the user's config should override the system config.

My major use case are proxies (see #93). Asking every user to configure them by hand is not optimal.